This document discusses planned and unplanned gaps in radiation therapy treatment schedules. Planned gaps are built into the schedule to account for tumor repopulation during weekends and holidays. Unplanned gaps negatively impact treatment outcomes by prolonging the overall time and allowing tumors to regrow. The effects of gaps depend on the prolongation length, tumor proliferation rate, and timing of the interruption. Corrections like increasing the dose or number of fractions are sometimes made to account for biological effects of treatment gaps.

1. Gap Correction in
Rt
Dr Kiran Kumar BR

2. GAP IN RT
Unplanned Gap
Planned Gap

3. • The gaps in all the fractionation schedules are called planned
interruptions/ gap
• All the fractions are planned to compensate for the tumor
repopulation during the non-treatment days
• So no need to worry about such gaps

4. • Any additional interruptions to the planned schedule is called
unintended interruptions/ gap.
• This results in prolongation of the treatment overall time
• Thus the treatment outcome is affected

5. Planned Interruptions
• Public holidays
• Weekends
• Machine service
Unplanned Interruptions
• Machine breakdown
• Radiotherapy reactions
• Patient unwillingness

6. Effects of interruptions
Planned gap between fractions
• Gaps are to achieve the aim of
RT
• Repair
• Redistribution
• Repopulation
• Reoxygenation
Unplanned gap between fractions
• Gaps affect Local control of
tumor & Reduces cure rate
• Repopulation of tumors
• Effects depends on
• Prolongation length
• Tumor proliferation rate (slow/
fast)
• Interruption timing

7. Prolongation length
•Effect in treatment outcome due to small gaps (1-2
days) is difficult to determine
•Gap of 1 week induce a loss of tumor control from
3-25%
•For breast cancer (post operative irradiation for 5
weeks) with a prolongation length of 1 week has a
increase risk of recurrence & death
•For squamous carcinoma the overall treatment time
should never exceed 56 days.

8. Tumor proliferation rate
• Patients with fast growing tumors will be adversely affected by interruptions
• For a slow growing tumors gap of 5 days doesn’t affect the treatment
outcome

9. Interruption timing
•Studies are being going on in this topic
•Events are classified as
• Gap arising in a short course (3-4 days)
• Gap arising earlier than 28 days
• Gap arising after 28 days
•Biological correction for each event is different
• Correction for gap arising after 28 days involves the
patient receiving high in short period which may increase
the risk of long term late effect.

10. 2 Gy, 19 days, 15
fractions
2 Gy, 12 days, 10
fractions
23 days
2 Gy, 25 fractions, 33 days
Part - I Part - II
Gap

11. GAP Correction
Increasing Dose
Increasing Fraction
Time Dose Fractionation
Factor
Biological equivalent dose

12. Time Dose Fractionation Factor

13. • TDF with no gap, A = 91.54
• TDF for part 1, B = 54.92
• Gap Factor, GF = 0.92
• Corrected TDF, C = B*GF = 50.52
• Remaining TDF, D = A – C = 41.02
• Remaining fractions = 11 fractions

14. Biological Equivalent Dose

15. •Thank You