Table 1.

Dose status for each ICRU volume and likely clinical outcome changes produced by CPT compared with MXT, where ↑ is an increased dose, = is the “equivalent” dose and ↓ is a reduced dose for the CPT. OTV is the volume of the body outside the PTV.

Dose Status	Tumour Control in GTV and {CTV+PTV}	{CTV+PTV} Side Effects	OTV Side Effects
GTV↑, {CTV+PTV}↑, OTV↓	Much better ***	Worse *	Better
GTV↑, {CTV+PTV}=, OTV↓	Better ***	Equal **	Better
GTV=, {CTV+PTV}=, OTV↓	Equal **	Equal **	Better
GTV=, {CTV+PTV}↓, OTV↓	Worse ****	Better	Better or worse

* In some instances reduced doses outside {CTV+PTV} may allow better radio-tolerance due to less late vascular insufficiency in the OTV. Also, this row condition may be acceptable in a non-essential tumour bearing tissue, or for a small volume of essential tissue with little risk of subsequent functional change. ** An equal outcome exists only if RBE is correct. *** In some instances (the radiosensitive tumour classes), tumour control will be very high with x-rays and there will be no gain in tumour control from dose escalation using any form of radiotherapy. These tumours may also have a lower RBE than 1.1, in which case the tumour control probability will be reduced if an RBE of 1.1 is used. **** Reductions in dose to CTV and PTV can occur due to range uncertainties and tissue movement, sometimes exacerbated by the use of scanned beams. The clinician may decide to extend the PTV and /or increase the local dose, depending on the tissue contained in the PTV and as a “trade-off” for gains in the OTV.